Metal pipelines, for example, oil or gas pipelines, are typically coated, both on the interior and the exterior, to protect the metal surface from corrosion or other undesirable effects. Coatings can include a wide variety of plastics, including polyethylene, polyurethane, or epoxy coatings. The exterior surface is typically coated to prevent corrosion due to contact with air or water, including salt water, for example, in transatlantic pipelines. The interior surface can also be coated, since contact with liquid material, for example, gas condensate, inside the pipeline can promote corrosion from within.
Typically, pipes used in pipelines are coated at the factory with a protective coating. The coating typically does not extend to the very ends of the pipes, since the metal must be exposed for attachment in the field. The pipes are then welded together at their ends, in the field, using girth welds; because these girth welds, and the surrounding surfaces, are typically exposed metal surfaces, they must then be coated, again, in the field. Exterior coatings are often in the form of a shrink wrap sleeve which is applied to the exterior surface of the exposed pipe, and shrunk into place using the application of heat. The interior surface of the exposed portion of the pipe surrounding (and including) the girth weld is often coated utilizing a preheated, liquid state plastic, utilizing a brush, or the application of a powder onto a pre-heated surface, said powder then melting to form a generally uniform coating.
As can be readily appreciated, once a pipeline is in place, access to the girth welds interspersed along its length can be problematic, especially on the interior of the pipe. Accordingly, internal pipe tractors have been developed, which can run along the inside of a pipe. The internal pipe tractors have wheels that run along the inside wall of the pipe, and can be controlled remotely, by an operator outside of the pipe. These internal pipe tractors can perform a variety of functions. For example, an internal pipe tractor can be hitched to an inspection cart, having lights and cameras for the visual inspection of the interior surface of a pipe. The inspection cart can identify minute defects in the pipe coating (known as holidays), for example, along the girth weld. Often these holidays are as small as a pin prick, but can be the point at which corrosion occurs.
Liquid coating carts are known in the art. For example, U.S. Pat. No. 5,181,962 describes a cart having an air compressor, a liquid coating material reservoir, and a rotating spray gun. The cart can be wheeled to a holiday/defect location using a standard internal pipe tractor. Then the rotating spray nozzle can be activated, such that liquid is sprayed through the spray gun utilizing the force of the compressed air, while the spray gun rotates. This coats the interior of the pipe with the liquid coating material. However, this type of liquid coating cart can only be used with liquid coating materials of low viscosity, and suffers from issues related to the clogging of the spray gun, which can be extremely inconvenient when the machine is several hundred feet away from the nearest operator.